Scattering States of Ionized Dopants Probed by Low Temperature Scanning Tunneling Spectroscopy

N type InAs(110) is investigated by scanning tunneling spectroscopy at low temperature. $dI/dV$ images at positive sample bias exhibit circular corrugations, which are caused by the scattering of electron waves at the attractive potential of ionized dopants. Normalizing the images by simultaneously recorded constant current images gives quasidirect access to the surface distribution of the corresponding energy selected scattering states. The normalized images are in quantitative agreement with a WKB model. The energy dependence of the scattering cross section can be estimated from the model.